1. Field of the Invention
The present invention relates generally to deep ultraviolet (DUV) lasers, and more specifically, it relates to DUV optically-pumped atomic vapor lasers (DUV-OPAVLs).
2. Description of the Related Art
As the feature size of silicon microelectronic integrated circuits (ICs) continues to decline in the quest for ever-higher speeds (from˜a micron five years ago to a projected ˜0.1 micron in the next few years), there continues to be an urgent need for practical laser sources with progressively shorter operating wavelengths from the present main production operating wavelength of 248 nm, and beyond the emergent advanced production operating wavelength of 193 nm. Such sources are needed to: 1) provide exposure radiation for patterning IC silicon wafers, in either a conventional reticle-based or a maskless-based exposure tool, 2) write photomasks of fine-line patterns for the manufacture of ICs, and 3) detect and classify wafer defects of progressively smaller size, assisting in the cost effective manufacture of ever-higher speed ICs. High-repetition-rate pulsed laser sources can be utilized for some of these applications, but continuous-wave lasers are often greatly preferred. The laser sources must be scalable in power to achieve sufficient process throughputs compatible with commercial production economics.
Production lithographic exposure laser sources require operating powers of up to several tens of watts which currently can be provided only by rare gas excimer lasers (KrF at 248 nm, ArF at 193 nm, and F2 at 157 nm). These lasers are necessarily pulsed lasers and are designed to operate at pulse-repetition-rates of up to several kHz. Each of these exposure lasers delivers its output radiation in the form of a hundred-kilowatt-level peak power pulses. The high peak intensity of these illumination sources can cause two-photon optical damage in optical elements of the lithographic imaging system. To avoid such effects a continuous wave exposure source of the same average power would be advantageous. To achieve the highest possible image resolution at a given exposure wavelength, it is advantageous to use an exposure source with extremely narrow spectral line-width (mitigating the effects of chromatic dispersion arising from optical elements in the optical train of the imaging system). In the case of the KrF and ArF excimer lasers, additional optical elements must be included in the laser resonator to effectively narrow the line-width of the output radiation, adding complexity and expense to these exposure lasers. Additionally, these excimer lasers require the use of corrosive and potentially dangerous halogen gases, also increasing the cost of ownership of these lithographic exposure sources. Thus, there continues to be a need for the development of continuous-wave, or of low-peak-power high-repetition-rate (i.e., >tens of kHz), narrowband lithographic exposure sources in the sub-250 nm spectral region, which can be scaled in output power to several tens of watts. Such a laser operating at lower power in the 0.1–1 watt range can also satisfy the needs for photomask writing and defect detection and identification. The effectiveness of these applications will significantly improve with the use of laser sources with operating wavelengths shorter than the presently commercially available sources at 244 nm and 257 nm. Again, continuous-wave or very high repetition rate laser source waveforms are highly desirable. Other applications such as 3-D rapid prototyping can also benefit from the availability of such lasers.
It can be appreciated that several sub ˜250 nm ultraviolet lasers have been known for years. These lasers are of several types. The first known such lasers were produced by high-current discharges in various atomic gases, such as argon, neon, and xenon. Generally, sub-250 nm laser transitions take place in the rare gas ions so that these lasers are generally quite inefficient (<<1%), are bulky, and require expensive power conditioning equipment More recently, sub-250 nm laser sources have been produced using non-linear conversion processes to convert radiation from a “drive” laser emitting at longer wavelengths into the shorter wavelength region. Because the efficiency of such non-linear conversion processes scale with the intensity of the longer wavelength drive laser, efficient sub-250 nm lasers generally utilize a pulsed drive laser having a peak intensity generally in excess of 10 MW/cm2. At such intensities, nonlinear optical materials tend to degrade due to optical damage. This is particularly so for nonlinear optical materials operating with output wavelengths sub-250 nm. Thus, sub-250 nm lasers produced using a nonlinear material to generate sub-250 nm radiation are not reliable, and complicated and expensive defensive measures must be adopted, such as periodically translating the nonlinear optical crystal transversely to the drive laser input beam to operate in an undamaged region of the crystal. Yet another approach to producing sub-250 nm lasers is to utilize a gaseous medium, such as xenon or mercury vapor, as a nonlinear conversion medium [1,2]. Again, to realize practically efficient conversion of drive laser radiation into sub-250 nm radiation, drive laser(s) providing high peak power pulses typically in excess of 10 MW/cm2 are utilized. It is also necessary to restrict the drive laser to operate in a single well-controlled frequency so as to achieve and maintain stable four-wave phase-matched conditions within the nonlinear mixing medium. While the optical damage problem of solid nonlinear optical materials is avoided, realization of efficient continuous-wave operation is generally precluded (as is operation even with low peak-power pulses and many tens of kHz pulse repetition rate).
Primarily argon ion lasers have been utilized for fine features defect detection in IC manufacturing. Defects on printed wafers have typically been detected utilizing argon ion lasers emitting 488 nm radiation and those on reticles (masks) have typically been detected utilizing argon lasers whose 488 nm and 514 nm radiation outputs have been frequency-doubled to 244 nm and 257 nm, respectively. While providing adequate power and spectral brightness, argon ion lasers are extremely inefficient (<0.01%), require extensive conditioned electrical power and active cooling, and are physically bulky. The stressing operating conditions within an argon laser generally limit the operating lifetime of a typical argon ion laser tube to <10,000 hours. Thus, there is a need to develop ultraviolet laser sources that are more than an order of magnitude more efficient (i.e., >1%), are much more compact, and require only comparably benign utilities.
In recent years, diode-pumped solid-state lasers have been developed to replace argon ion lasers with performance features that are superior to the argon ion laser. Generally, these lasers comprise a diode-pumped solid-state crystal (such as Nd: YAG or Nd: YVO4) emitting “fundamental” radiation in the near infrared spectral region (i.e., λ˜1064 nm), and one or more harmonic nonlinear optical (NLO) crystal converters. The NLO elements convert the fundamental IR radiation into radiation of shorter “harmonic” wavelengths: λ/2, λ/3, λ/4, etc (i.e., 532 nm, 355 nm, 266 nm, respectively). Practically efficient harmonic conversion requires of the NLO crystal that:                its birefringence is such that the NLO conversion process is “phase-matched” (i.e., the indices of refraction at both fundamental and harmonic wavelengths are equal);        its nonlinearity is adequately large;        it is adequately transparent at all operating wavelengths; and        its intensity threshold for optical damage substantially exceeds that of the drive intensity needed for efficient NLO conversion.        
A decade-long sustained search for practical NLO crystals has resulted in the identification and development of only a few NLO crystals meeting the requirements itemized above, especially for harmonic wavelengths lying in the ultraviolet (UV) and deep ultraviolet (DUV) spectral regions. Among these are lithium borate (LBO), beta-meta-borate (BBO), and cesium-lithium borate (CLBO). The former two NLO crystals have now found extensive commercial service in laser sources operating at wavelengths down to ˜244 nm. In attempts to utilize these crystals at even shorter wavelengths, problems with conversion efficiency and operating lifetime decreases substantially due to a narrowing between the required drive intensity for practical conversion efficiency and the threshold intensity for optical damage. Thus, there is a need to find a practical alternative means for converting the radiation of laser sources operating at wavelengths of ˜266 nm or longer (e.g., harmonically converted DPSSLs) to laser radiation of wavelengths shorter than ˜250 nm.
Very recently, a continuous-wave 198.5 nm laser source has been demonstrated based on sum frequency mixing in a CLBO nonlinear crystal of a 244 nm frequency doubled argon ion laser and a 1064 nm Nd:YVO4 laser. Because of the very low efficiency of the argon ion laser, this laser scheme is not regarded as practical for commercial use. A future embodiment would replace the 244 nm argon ion laser with a 244 nm source based on an optically-pumped semiconductor laser emitting at a fundamental wavelength of 976 nm as the primary drive laser [3]. This drive laser would be converted to 244 nm radiation by resonant-cavity fourth-harmonic generation in nonlinear optical crystals; this radiation would then be coherently mixed with radiation from a separate diode-pumped solid state laser source emitting near one micron using CLBO as the mixing nonlinear optical crystal. The cost and performance of this type of source is burdened by the requirements of phase-matching all drive and output waves (in resonant optical cavities) and optical damage in CLBO at wavelength below 200 nm.
The present invention provides a practical means for the efficient conversion of laser sources operating at selected wavelengths longer than ˜250 nm into a laser source operating at ultraviolet wavelengths shorter than ˜230 nm, without utilizing or subjecting a NLO crystal to irradiation at the <230 mm wavelength of the laser output, where NLO crystal converters tend to lose their practical effectiveness. The present invention teaches how atomic vapors of the group IIB elements of the periodic table of the elements (mercury (Hg); cadmium (Cd); and zinc (Zn)) can be utilized to efficiently “sum” the output power of certain “drive” or pump lasers whose output wavelengths match certain transition wavelengths of the vapor atoms, when the vapor is mixed with an appropriate buffer gas (or buffer gas mixture) and placed within a laser resonator cavity that has sufficiently high reflectivities (or Q-factors) at the appropriate wavelengths. This type of laser device is referred to herein as a deep ultraviolet optically-pumped atomic vapor laser, or DUV OPAVL. The power summing process scales with drive laser intensities and, due to the large strengths of the atomic transition dipoles involved in the power summing process, the operating drive intensities are several orders of magnitude lower than those found in conventional lasers using NLO crystal converters (i.e., 10's of kW/cm2 vs. 10's of MW/cm2). At the same time, there is no requirement for “phase-matching” the input and output waves, since the conversion process itself is incoherent. Analysis also shows that, because of their relatively large transition cross-sections and relatively low corresponding saturation intensities, these optically-pumped group IIB atomic vapor lasers can efficiently generate laser power with a purely continuous-wave temporal waveform, or with a train of relatively low-peak-power pulses at repetition rates of many tens of kHz. Therefore, the life-limiting optical damage processes present in conventional NLO wavelength converters at wavelengths shorter than ˜230 nm are absent in this type of radiation converter, providing for long-lived power conversion in the DUV spectral region below ˜230 nm. This novel class of optically-pumped group IIB atomic vapor lasers differs essentially from the teachings of Ghaffari [4] who describes a mercury vapor based high power light system for medical applications. It also differs fundamentally from the 546.1 nm mercury vapor laser taught by Siegman, et al. [5–7], by Djeu [8] and by Znamenskii [9], which teach mercury vapor lasers that are optically-pumped by an rf discharge in mercury vapor, where the 546.1 nm laser transition does not terminate on the mercury atom ground level, and does not operate in the DUV.